Project Summary The broad, long-term goal of our program is to advance the knowledge of virus-host cell interactions by characterizing cellular cofactors essential for hepatitis C virus (HCV) infection. HCV infects 170 million people worldwide and is major cause of hepatocellular carcinoma. Understanding how this virus interacts with the host is of critical importance to the development of diagnostics, antiviral drugs, and a prophylactic vaccine. Our previous studies demonstrated that HCV infection of cultured hepatoma cells is critically dependent on a cellular protein, cyclophilin A (CyPA), and that ablating the function of this protein can not only prevent new infections but also suppress an existing infection. In addition, CyPA is a principal mediator of HCV resistance to cyclosporine A (CsA) and its derivatives, which inhibit HCV replication with an undefined mechanism and are currently being evaluated in clinical trials as candidate anti- HCV drugs. The experiments proposed here will investigate the molecular mechanisms that determine the essential cofactor function of CyPA, the action of CsA, and the related CsA resistance. The following experiments will be performed: (1) CsA and small interfering RNA directed at CyPA mRNA will be used to identify the specific function of HCV replicase that requires CyPA as a cofactor. A detailed understanding of why and where the virus needs this cellular chaperone to survive will offer new perspectives on the replication strategy of HCV. (2) Biochemical assay and mutagenesis will be used to characterize the critical interaction between CyPA and the viral replicase, which represents a novel structural interface of virus-host cell interaction that maybe disrupted for therapeutic purposes. (3) Macromolecular interaction and reverse genetics will be employed to dissect the mode of action for CsA and molecular basis of CsA resistance with the ultimate goal of predicting and circumventing that resistance. The results of the proposed studies will not only provide significant insights into how highly successful parasites such as human viruses hijack host cell machineries to replicate efficiently but also reveal new therapeutic targets for antiviral intervention.